1. Field of the Invention
The present invention relates to a method and apparatus for applying photoresist material on a semiconductor wafer. More specifically, the present invention relates to a method and apparatus for obtaining a uniform photoresist layer on a wafer in preparation for a subsequent photoetching process.
2. Description of the Related Art
Semiconductor devices are manufactured through a series of processes. One such process is the deposition of a photoresist material on the surface of a semiconductor wafer in preparation for a subsequent etching process. Many methods are available for applying photoresist material to wafers but they all have certain drawbacks.
Available methods include an immersing method, a spraying method, and a rolling method, which are usually adapted for use with liquid photoresist materials. Because the back sides of the wafers may be coated with the photoresist material during the immersion and spraying methods, these two methods are inappropriate for semiconductor devices. The rolling method is generally used for printed circuit boards but it is inappropriate for semiconductor devices because it is difficult to control the thickness of the photoresist layer using this method. Accordingly, a spin-coating method has been introduced to overcome the deficiencies of the above methods.
The spin-coating method is one of the better methods for precisely controlling the thickness of a photoresist layer. For example, if the thickness of the photoresist layer is 0.5 .mu.m, the thickness variation would be less than about 10% using the spin-coating method. A rotating device, also known as a spinner, is usually used in such a spin-coating method. A conventional spin-coating apparatus is shown in FIG. 1.
Referring to FIG. 1, wafers 10 are fixed on vacuum chucks 7 by a vacuum suction force and are rotated by a spinner 5. A bowl 8 is installed above the vacuum chucks 7 to protect photoresist dispensed to the wafers 10 from scattering when the wafers 10 are rotated. An exhaust port 9 is formed at a predetermined position on the side of the bowl 8. A catch cup 11 is installed at the bottom of the apparatus, where the photoresist material dropping from the bowl 8 is collected.
The operation of the spin-coating apparatus is as follows. First, an elevator 12 is operated to elevate a cassette 2 loaded with wafers 10 to the level of the vacuum chucks 7. The wafers 10 in the cassette 2 are then successively transferred to the vacuum chucks 7. Filtered nitrogen 6 is supplied to the surface of the wafers 10 via a conduit to blow away the particles on the wafers 10.
Next, a primer 3 is applied to the wafers 10 via conduit to enhance the adhesion of the photoresist 4 to the wafers 10. The primer 3 may be applied to the wafers 10 by spinning or evaporating. An evaporation priming process or a vacuum baking evaporation priming process may also be used.
When the priming process is completed, the photoresist 4 is dispensed on the wafers 10 via a conduit. Two pumping systems may be used for the dispensing process. In the first pumping system, the photoresist contained in a vessel is pushed out of the vessel through a tip at the end of the conduit by using dry nitrogen gas to increase the pressure inside the vessel. The other pumping system is a diaphragm system in which a bellows is used to compress and push out photoresist through the tip of the conduit. The particular thickness of the photoresist layer coated on the wafers 10 does not have much of an influence on the process parameters. However, when an insufficient amount of photoresist is supplied, the surfaces of the wafers 10 cannot be completely coated with a photoresist layer. On the other hand, if too much photoresist is supplied, the back sides of the wafers may also be coated with the photoresist, which is undesirable because it affects wafer alignment.
The photoresist spin-coating process is carried out using a dynamic dispense cycle. In the dynamic dispense cycle, photoresist is applied to wafers 10 while the wafers 10 are rotated slowly at about 500 RPM. After applying the photoresist, the wafers 10 are rotated at a higher rate so that the photoresist can be uniformly spread out on the wafers 10.
After the deposition of the photoresist, the wafers 10 are unloaded from the vacuum chucks 7 and stored in the cassette 2 for transfer to an oven for subsequent processing, i.e., a soft baking process.
The spin-coating process using the spinner 5, however, suffers a drawback. Specifically, it is difficult to obtain a uniform thickness layer of photoresist on the wafers 10. The centrifugal force, which is generated by the rotation of the spinner 5, causes the photoresist to move away from the axis of the rotation. Because the magnitude of the centrifugal force depends on the distance from the axis of rotation, the movement of photoresist material varies with position on the wafer. This results in an uneven photoresist coating layer on the wafers 10.
Several approaches and apparatuses have been disclosed to overcome such deficiency. One such apparatus is disclosed in Korean Patent Publication Number 94-10497, and includes an inner bowl between a spinner and an outer bowl enclosing the spinner. The inner bowl is simultaneously moved up by an elevating means when the wafers stop rotating. When the inner bowl moves up and encloses the wafers, the movement of the air at the center of the wafers is the same as at the circumference of the wafers.
Korean Patent Publication Number 93-8859 discloses a spin-coating method for enhancing the uniformity of photoresist thickness on a wafer. The method includes three additional steps that are performed after dispensing photoresist on the surface of the wafers. First, the wafers are rotated at a first velocity for a designated amount of time. Then, the wafers are rotated at a second, faster velocity for another designated amount of time. During the rotation at the second velocity, the wafers are intermittently rotated for 1-2 seconds at a third velocity which is faster than the second velocity. The third rotation step is preferably repeated more than two times during the second rotation step. As a result, winding surfaces are formed on the wafers in association with the rotations at the different velocities.
However, the apparatus and methods described above fail to consider that a cause of the uneven photoresist thickness is the radially varying centrifugal force, which yields a radially varying thickness of photoresist material. In other words, the dispensed photoresist material is influenced by the varying centrifugal force as the spinner rotates and the material spreads over the whole surface of the wafer. The magnitude of the centrifugal force influencing the photoresist material is proportional to the square of the radius at which the material is located on the rotated wafer. Therefore the level of centrifugal force varies with the position on the wafer. For example, the force is almost zero at the center of the wafers and is at a maximum at the circumference of the wafers. Such differences in the centrifugal force result in differences in the thickness of the photoresist layer; inevitably the thickness of the photoresist layer at the center of the wafer will be different from that at the other areas of the wafer. Moreover, a wafer having a larger radius (e.g., 12 inch wafers which are still under development) will have a greater difference in the center-to-edge thickness of a photoresist layer than the 8 inch wafers that are currently used. The difference in thickness can be controlled to some extent by adjusting the spinning velocity of the spinner and modifying the process variables in association with the viscosity of the photoresist. However, the difference between the thicknesses at the center of the wafer where the centrifugal force is zero compared to the thickness at the other areas of the same wafer cannot be fundamentally eliminated.